Electroluminescent devices are actively researched among the other flat panel display devices because they may emit various colors of light, may be easily patterned and made into thin films, and have low direct current driving voltage and high luminous efficiency.
In particular, organic electroluminescent devices (OLEDs) do not require a separate light source unlike a conventional thin film transistor liquid crystal display devices (TFT-LCDs), thereby reducing the volume and weight of the devices, and may be driven with a low voltage compared to a plasma display panel. Due to those advantages, organic electroluminescent devices are expected to be widely used in the future.
Since an organic electroluminescent device is made of a plurality of organic thin films, it may react with moisture, oxygen and the like so that the lifespan of the device may be shortened. For this reason, a cap for protecting the organic electroluminescent device is covered thereon in various manners so as to prevent the organic electroluminescent device from being exposed to the outside, and such a process is referred to as an encapsulation process. Conventionally, inorganic materials such as glass and ceramic have been mainly used for an encapsulation film of the organic electroluminescent device.
An encapsulation film made of an inorganic material such as glass or ceramic does not have problems such as thermal deformation and the like. However, the glass material has a low thermal conductivity, and thus heat generated during the use of the organic electroluminescent device is not released smoothly, causing degradation of electroluminescent layers and reduction in lifespan of the organic electroluminescent device.
Accordingly, metallic encapsulation films, particularly made of Invar, have been recently used in some cases in order to solve the problems caused by the encapsulation films made of inorganic materials. Invar is a nickel-iron alloy and so named because it is invariant to temperature changes, i.e. the thermal expansion coefficient thereof is about one millionth at a temperature below 100° C. and nearly zero at room temperature. In general, Invar alloy is used as a material for components of a machine such as a precision machine, optical machine and watch, wherein the change in dimensions of such components due to temperature variation may cause errors.
Meanwhile, in order to perform a process of encapsulating an organic electroluminescent device using an encapsulation film made of Invar, a technique for facilitating the encapsulating process is required.